Optical technology has many characteristics such as capability of high-speed processing at a frequency of light (speed is high because of high frequency), capability of spatial information processing, and capability of phase processing. For this reason, the optical technology has been researched, developed, and put into practical use in a wide range of fields such as communications, measurement, and processing.
Such optical technology uses a high-precision objective lens to focus a light beam.
In accordance with a recent growing demand particularly for image recording devices utilizing light, technique aiming to increase the storage capacity of an optical information recording medium has become quite important. In order to increase the storage capacity of an optical information recording medium, the quality of the recording medium needs to be improved, and what is more, a beam spot needs to have a smaller diameter, namely the objective lens needs to sufficiently focus the beam spot.
As is well known, the diameter of the beam spot is proportional to the wavelength of light, and is inversely proportional to the NA (Numerical Aperture) of the objective lens. In other words, it is necessary to either shorten the wavelength of light or increase the NA of the objective lens, in order to focus the beam spot having a smaller diameter.
In order to shorten the wavelength of light, a blue laser diode and a blue or green SHG laser have been recently developed. On the other hand, in order to increase the NA of the objective lens, a higher density has been achieved in DVD (Digital Versatile Discs) whose NA is 0.6 compared with CD (Compact Disc) whose NA is 0.45. Further, Japanese Unexamined Patent Publication No. 123410/1998 (Tokukaihei 10-123410, published on May 15, 1998) discloses an optical pickup device aiming to have an increased density. This optical pickup device uses a 2-group lens in which two lenses are combined, so that the objective lens has an NA of 0.85.
The optical pickup device which uses the objective lens having a high NA as described above is required to correct (i) unevenness in thickness of a light transmission layer of an optical recording medium and (ii) a spherical aberration which occurs in multi-layer recording. For example, Japanese Unexamined Patent Publication No. 143303/2001 (Tokukai 2001-143303, published on May 25, 2001) discloses a liquid crystal element which corrects the spherical aberration.
The liquid crystal element of the foregoing Publication is arranged such that liquid crystal is sandwiched between electrodes which are respectively formed on glass substrates. In order to correct the spherical aberration, the liquid crystal element applies a voltage to the electrodes so as to change the alignment of the liquid crystal. This changes a refractive index of the liquid crystal, thereby forming a phase distribution.
As an example, the following describes a system in which the objective lens has an NA of 0.85 and the light transmission layer of the optical recording medium has a thickness of 0.1 mm. In the event where a light transmission layer with a thickness of 0.115 mm is used, a phase distribution as shown in FIG. 8 is imparted to the liquid crystal element so as to correct the spherical aberration caused by the increased thickness. This phase distribution is for a spherical aberration corresponding to the increased thickness (+15 μm) of the light transmission layer, and it is required to correct a spherical aberration remaining on a best image plane after focusing. In other words, the phase distribution is formed to reduce spherical aberration, and it is required to correct a spherical aberration that remains after the objective lens was moved in a direction of the optical axis in the focus operation that is carried out to match a recording-reproducing surface with the best image plane on which a wavefront aberration is the smallest.
FIG. 9 shows a relationship between radius r of the liquid crystal element and the second derivative of the phase distribution, wherein the second derivative indicates the rate of change (rate of fluctuation) in the slope of the phase. As shown in FIG. 9, the phase distribution has two inflection points. Further, the graph of FIG. 9 has considerably large values at outermost portions of the effective light beam. Here, imparting such a phase distribution to the liquid crystal element does not cause a problem when the central axis of the liquid crystal element is aligned with the central axis of the objective lens. However, when the central axis of the liquid crystal element and the central axis of the objective lens are misaligned with each other even slightly in the radius direction of the liquid crystal element and the objective lens, the wavefront of incident light on the objective lens is greatly disturbed, thereby causing a large aberration.
Therefore, the foregoing arrangement has a problem that the characteristics largely deteriorate due to the center misalignment of the liquid crystal element with the objective lens. Further, the liquid crystal element and the objective lens need to be precisely aligned with each other in order to prevent deterioration of the characteristics, thereby forcing complicated work on a user. Further, when the objective lens mounted on an actuator is moved in a direction perpendicular to the optical axis and perpendicular to the tracking direction in order to carry out a tracking operation, the center misalignment occurs such that the optical axis of the light beam entering the objective lens is misaligned with the central axis of the objective lens. Because such center misalignment is not allowable, it is also necessary to mount both the objective lens and the liquid crystal element together on the actuator.
Further, in order to allow linearly polarized light to enter the liquid crystal element, it is necessary to provide a quarter-wave plate at a position closer to the optical recording medium than the liquid crystal element, namely on the actuator. This increases the weight of parts mounted on the actuator (weight of moving parts), and thus prevents high-speed driving, making it difficult to increase the speed of recording and reproducing.
Another problem of mounting the liquid crystal element on the moving part (actuator) is that it becomes difficult to route lead wires for applying a voltage to the liquid crystal element, or to position a flexible substrate.
In view of the foregoing problems, the present invention has an objective to provide an optical pickup device for reducing the aberration caused by position misalignment of the liquid crystal element with the objective lens, even if the objective lens has a high NA.